Blades of modern wind turbines are quite long—they are up to 45 meters long or even more. Thus their shape and their characteristics need to be optimized, especially to prevent that turning blades hit the tower while the wind turbine is in operation.
Especially the deflection or bending of the blade near its tip-end needs to be known to prevent those damages.
It is known to attach strain gauges on the blade-surface. They are mainly used for test purposes to gather a certain knowledge about the blade when it is stressed.
The installation of gauges and the installation of their electrical cabling needed is expensive, especially asides a wind turbine which is in operation. The equipment (especially the cabling) is exposed to lightning strikes, thus the gauges are mainly used for time-limited test purposes.
It is known to use distance-measurement-systems to measure blade-deflections. Parts of the system (like a reflector for wireless transmitted optical signals) are arranged inside a cavity of the blade, while other parts of the system (like an optical transmitter and/or optical receiver) are arranged inside the hub of the wind turbine.
Only a certain blade-deflection can be measured by theses systems due to the location of the system-parts. The deflection of the tip-end of the blade can only be approximated as the cavity is not elongated towards the tip-end of the blade.